The present invention relates to a steam turbine and, more particularly, it relates to steam turbine designed to achieve a high efficiency by improving the nozzle box arrangement in the steam inlet section.
Generally, a steam turbine comprises a rotatable turbine rotor, moving blade stages, a casing and nozzle diaphragms. The casing and the nozzle diaphragms constitute as a stationary section. The rotor is rotatably provided in the casing. The nozzle diaphragms are arranged substantially coaxially with the turbine rotor, supported on the casing. The moving blade stages are provided on the turbine rotor so as to rotate together with the turbine rotor. Each of the moving blade stages comprises a plurality of moving blades arranged in the circumferential direction of the turbine rotor.
Each of the nozzle diaphragms comprises a plurality of turbine nozzles arranged in the circumferential direction relative to the turbine rotor and arranged at the upstream side of one of the moving blade stage. A pair of a nozzle diaphragm and a moving blade stage provided at the upstream side of the nozzle diaphragm forms a turbine stage. An ordinary steam turbine has a plurality of turbine stages.
More specifically, nozzle diaphragms, a turbine rotor and moving blade stages are substantially coaxially arranged in the casing. The steam led to a nozzle diaphragm passes through a plurality of turbine nozzles of the nozzle diaphragm and change its flowing direction. Then, the steam flowing out from the nozzle diaphragm is led to a moving blade portion of a moving blade stage that forms a pair with the nozzle diaphragm. The steam drives the moving blade stage and the turbine rotor as it passes between the plurality of moving blades of the moving blade stage.
As pointed out above, an ordinary steam turbine has a plurality of turbine stages. The steam that passes through one turbine stage is led to an adjacent turbine stage. More specifically, a plurality of moving blade stages are provided on the turbine rotor, separated from each other in the axial direction. The nozzle diaphragms are arranged in the casing so as to be placed between the moving blade stages in the axial direction of the turbine rotor. The moving blade portions of a plurality of moving blade stages and the turbine nozzle portions of a plurality of nozzle diaphragms form a steam passage.
Especially, for a high pressure turbine, a nozzle box is provided in the casing to lead the steam introduced in the casing to the turbine nozzles of the first stage, which constitute as a part of the steam passage. Known nozzle boxes include one described in Japanese Patent Application Laid-Open Publication No. 03-066484, the entire content of which is incorporated herein by reference.
Like the casing, the nozzle box constitutes as the stationary section. The nozzle box comprises a plurality of turbine nozzles of the first stage, which are arranged in the circumferential direction, provided at the outlet side of the nozzle box. In other words, the nozzle box and the nozzle diaphragm of the first stage (e.g. the first stage nozzle diaphragm) are arranged integrally and the steam introduced into the nozzle box is led to the steam passage, that includes the first moving blade stage that forms a pair with the first stage nozzle diaphragm provided with the nozzle box.
FIGS. 8 and 9 are schematic axial cross-sectional views of a known steam turbine having a nozzle box. FIG. 8 is a schematic axial cross-sectional view along a vertical direction and FIG. 9 is a schematic axial cross-sectional view along an angle inclined relative to the vertical direction by 45°.
The steam turbine 1 has a casing 2, a turbine rotor 3 rotatably arranged in the casing 2, a nozzle diaphragms 4a1, 4a2, 4a3, . . . that are rigidly secured to the casing 2. The casing 2 includes an outer casing 2a and an inner casing 2b. 
A plurality of moving blade stages 3a1, 3a2, 3a3, . . . , are arranged on the turbine rotor 3, which is a rotating section of the steam turbine 1, in the axial direction from the upstream side to the downstream side. Each of the moving blade stages 3a1, 3a2, 3a3 has a plurality of moving blades, the plurality of moving blades of the moving blade stages being denoted respectively by 3b1, 3b2, 3b3, . . . , and rotating force is generated as steam flows, passing through the moving blades 3b1, 3b2, 3b3, . . . .
Nozzle diaphragms 4a1, 4a2, 4a3, . . . that are supported by the inner casing 2b are arranged between the moving blade stages 3a1, 3a2, 3a3, . . . such that they are substantially coaxial and separated from each other in the axial direction. A pair of the nozzle diaphragms 4a1, 4a2, 4a3, . . . and the moving blade stages 3a1, 3a2, 3a3, . . . , respectively, constitutes a turbine stage. A plurality of turbine nozzles 4b1, 4b2, 4b3, . . . are provided in the circumferential direction, respectively, with the nozzle diaphragms 4a1, 4a2, 4a3, . . . .
The nozzle diaphragms 4a1, 4a2, 4a3, . . . are supported by the casing 2 so as to constitute a stationary section of the steam turbine 1. The steam flow flowing through between the plurality of nozzle blades 4b1, 4b2, 4b3, . . . arranged in the circumferential direction is changed its flowing direction so as to be led to the moving blades 3b1, 3b2, 3b3, . . . of the moving blade stages 3a1, 3a2, 3a3, . . . of the pairs. The flow path of the steam including the portions of the turbine nozzles 4b1, 4b2, 4b3, . . . of the nozzle diaphragms 4a1, 4a2, 4a3, . . . and the portions of the moving blades 3b1, 3b2, 3b3, . . . of the moving blade stages 3a1, 3a2, 3a3 constitute as steam passage 8. The steam led to the steam turbine 1 flows through the steam passage 8 from an upstream side to a downstream side.
The steam turbine 1 is provided with a steam inlet pipe 7 and a nozzle box 5 that constitutes as members for introducing steam into the steam passage 8. The nozzle box 5 is a pressure vessel that deals with high temperature and high pressure steam. An inlet section of the nozzle box 5 is connected to the steam inlet pipe 7. A steam outlet section, namely, outlet section, of the nozzle box 5 is integrally provided with the first stage nozzle diaphragm 4a1 and the plurality of turbine nozzles 4b1 that are arranged in the circumferential direction.
The nozzle box 5 is rigidly secured to the casing 2 by a support member 6 arranged on the inner casing 2b. The plurality of first stage turbine nozzles 4b1, integrally arranged in the circumferential direction at the outlet section, serves as the first stage nozzle diaphragm 4a1. The nozzle box 5 is arranged substantially coaxial with the turbine rotor 3.
Thus, the steam flowed into the nozzle box 5 from the steam inlet pipe 7 is then led to the first stage nozzle diaphragm 4a1 that constitute as a part of steam passage 8. The steam led to the steam passage 8 expands as it passes through between the turbine nozzles 4b1, 4b2, 4b3, . . . and the moving blades 3b1, 3b2, 3b3, . . . and the thermal energy is converted into kinetic energy to drive the moving blade stages 3a1, 3a2, 3a3, . . . and the turbine rotor 3.
Note that the support member 6 is a member for supporting the nozzle box 5 in the inner casing 2b. The support member 6 is not arranged entirely along the nozzle box 5 in the circumferential direction as seen in FIG. 9.
The nozzle box 5 is arranged in a space formed between the inner casing 2b and the turbine rotor 3. The pressure of the space around the nozzle box 5 is substantially equal to the pressure of the steam passage 8 near the outlet of the first moving blade stage 3a1.
More particularly, in the steam turbine 1 as shown in FIG. 9, a part of the steam flowing out from the first stage nozzle diaphragm 4a1 of the nozzle box 5 does not flow along the steam passage 8 into the first moving blade stage 3a1, which outputs rotation energy converted from thermal energy. The steam, which does not flow along the steam passage 8 at the downstream side of the first stage nozzle diaphragm 4a1 of the nozzle box 5, leaks to the space around the nozzle box 5 and bypasses to the downstream side of the first moving blade stage 3a1 via an outer circumferential side of the nozzle box 5 (e.g. a space between the nozzle box 5 and the inner casing 2b), as indicated by dotted arrows in FIG. 9. This problem becomes particularly significant in a turbine having a large degree of reaction where the pressure difference between the outlet of the first stage turbine nozzles 4b1 and the outlet of the first moving blade stage 3a1 is large.
Additionally, in the known steam turbine 1, the pressure of the space around the nozzle box 5 is substantially equal to the pressure at the outlet of first moving blade stage 3a1, which has a large pressure difference with that of the steam flowing into the nozzle box 5. Therefore, when the steam conditions such as the temperature and the pressure of the steam flowing into the steam turbine 1 are raised in order to improve the efficiency of the steam turbine 1, further studies are necessary including the wall thickness of the nozzle box 5 and the materials suitable for the nozzle box 5 such as heat-resistant steel. The net result will be a raised cost of such a steam turbine 1.